We have established that heat-killed Brucella abortus conjugated to a V3-loop peptide from HIV-1 MN can elicit neutralizing antibodies and CTL in normal and mice depleted of CD4+ T-cells, systemically and at mucosal surfaces. The antibodies elicited in mice lacking CD4+ T-cells were mainly of the IgG2a isotype. Sera from these mice were more efficient than sera containing predominantly IgG1 in neutralizing HIV-1. Furthermore this conjugate can induce neutralizing antibody (systemic and mucosal) and cellular responses in Rhesus macaques. Secretions from several mucosal sites including vagina exhibited ant-HIV neutralizing activity. Previously, we have shown that a strong Th1 stimulus, in the form of Brucella abortus, can switch Th2 to Th1 responses and inhibit IgE induction. These studies were performed in adult mice. Down-regulation of the Th2-like response induced by ovalbumin-alum (OVA/ALUM)by heat-killed Brucella abortus (BA) was not reversed by anti-IL-12 antibody treatment or in IFNg KO mice, suggesting that induction of Th1 cytokines was not the only mechanism involved in the BA-mediated inhibition of the Th2 response to OVA/ALUM. A study was performed to determine whether an alternative pathway involved alteration in expression of costimulatory molecules. B7.2, but not B7.1, was up-regulated on mouse non-T and T-cells following BA. Surprisingly, BA induced down-regulation of CD28 and up-regulation of B7.2 on murine CD4+ and CD8+ T cells. These effects on T-cells were maximal for CD28 and B7.2 at 40-48 h, and were not dependent on IL-12 or IFN-g. On the basis of these results we propose that the IL-12/IFNg- independent inhibition of Th2 responses to OVA/ALUM are secondary to the effects of BA on expression of costimulatory molecules on T cells. We suggest that down-regulation of CD28 following activation inhibits subsequent differentiation of Th0 into Th2 cells. In addition, decreased expression of CD28 and increased expression of B7.2 on T cells would favor B7.2 interaction with CTLA-4 on T cells and this could provide a negative signal to developing Th2 cells. Mouse IgG2a is a complement-fixing isotype. In order to study whether human complement-fixing antibodies are more potent in neutralizing virus, polyclonal antibody has been obtained from individuals with high titer against respiratory syncytia virus (RSVIG). This product was passed over protein A columns and purified into IgG3 and enriched for IgG2, IgG1 and IgG4. Further purifications will involve immunoaffinity columns. Preliminary data suggest that human IgG1 is more effective than IgG3 in neutralizing RSV.